Spark discharge device



April 5, 1966 J. A. HULL 3,244,935

SPARK DISCHARGE DEVICE Original Filed Oct. 31, 1957 6 Sheets-Sheet lJOSEPH A. HULL INVENTOR BY W% ATTORNEY April 5, 1966 J. A. HULL3,244,936

SPARK DISCHARGE DEVICE' Original Filed 001:. 31. 1957 6 Sheets-Sheet 2JOSEPH A.HULL

INVENTOR.

ATTORNEY April 5, 1966 J. A. HULL SPARK DISCHARGE DEVICE 6 Sheets-Sheet5 Original Filed Oct. 31. 195'? IN ENTOR.

JOSEPH AHULL ATTORNEY April 1966 J. A. HULL 3,244,936

SPARK DISCHARGE DEVICE Original Filed Oct. 51. 1957 6 Sheets-Sheet 4.

m In v E g E a) (O k 9 I N U) N 9 A w 8 m I qo o 9 3 J JOSEPH A. HULLINVENTOR.

ATTORNEY April 1966 J. A. HULL 3,244,936

SPARK DISCHARGE DEVICE Original Filed Oct. 31. 1957 6 Sheets-Sheet 5JOSEPH A.HULL

ATTORNEY April 5, 1966 J. A. HULL 3,244,936

SPARK DISCHARGE DEVICE Original Filed Oct. 31. 1957 6 Sheets-Sheet 6JOSEPH A. HULL ATTORNEY United States Patent 3,244,936 SPARK DISCHARGEDEVICE Joseph A. Hull, Danvers, Mass, assignor, by mesue assignments, toUuiiectron, Inc., Cambridge, Mass, a corporation of Massachusetts(Eriginal application Oct. 31, 1957, Ser. No. 693,570, now Patent No.3,041,936, dated July 3, 1962. Divided and this application Apr. 3,1962, Ser. No. 193,031

3 Claims. (Cl. 315237) This invention relates to control devices andmore particularly to spark discharge devices especially useful in thephotographing of objects moving at high speed, such as missile-s undertest. This is a division of my application Serial No. 693,570, filedOctober 31, 1957, now Patent No. 3,041,936.

The invention has particular application to studies wherein re-entryconditions of intercontinental ballistic missiles are aerodynamicallysimulated. It is customary for such simulation studies to be made inconjunction with a model of the missile which is projected at high speedin a ballistic range or tested in high speed wind tunnels. As techniquesfor obtaining high velocities have been improved, the demands oninstrumentation required for measuring associated parameters have alsoincreased. High speed photographic techniques used to record theposition of models in flight, along with a history of the flow patternsurrounding the model, are particularly useful. If one considers aprojectile travelling at a velocity of 20,000 feet per second, beingphotographed on film having a resolution of 20 lines per mm. at amagnification factor of /2, it will be apparent that it is necessary tolimit the exposure time of the film to .01 microsecond in order toprevent image blur caused by motion of the projectile.

Briefly, the present invention comprises an improved spark dischargedevice which enables a light source to be synchronized with an outputdevice such as a pulse generator which is capable of delivering to aKerr cell extremely large, high speed voltage pulses for operating thecell as a high speed optical light valve or shutter. The spark dischargedevice produces the pulse for the Kerr cell by enabling a pulsegenerator to discharge its energy through a spark gap which may also bearranged to produce radiant energy for illuminating the subject beingphotographed. Special circuits are provided for triggering the sparkdischarge so that the subject will be photographed when it is Within thefield of view of an associated camera. 7

It is also within the purview of the invention to provide an improvedmultiple trigger spark discharge device and associated circuit fortaking several pictures of the same subject at short time intervals.This circuit produces trigger pulses for initiating spark discharge atpre-selected known time intervals.

Through the improvements made in the spark discharge devices,illumination means, and trigger circuits, it is possible to use a Kerrcell of unusually large proportions suitable for photographing highvelocity subjects with a relatively large magnification factor and toassure automatic and perfect synchronization of the illumination of thesubject with the operation of the Kerr cell.

In view of the foregoing it will be understood that it is a generalobject of the invention to provide improved means for taking high speedphotographs, particularly photographs of subjects moving at extremelyhigh velocities.

A further object of the invention is the provision of an improved sparkdischarge device which is capable of controlling the pulse generatorwhile simultaneously providing illumination of the subject :beingphotographed. More particularly, a spark discharge device is providedwhich 3,244,936 Patented Apr. 5, 1.966

ice

can be charged with inert gases under pressure in order to preventpremature ionization of the spark gap and also to increase illuminationefiiciency.

Another object of the invention is to provide a multiple spark dischargedevice and means for applying separate trigger pulses to control thesequence and timing of the multiple discharges.

Another object of the invention is the provision of special circuits foroperating both the single and multiple spark discharge devices wherebyan associated Kerr cell can be used as a shutter for taking eithersingle or multiple exposure photographs of high velocity subjects.

Other objects are as follows:

(1) Provision of an improved light screen for operating the triggercircuits so that Kerr cell operation will occur while the subject iswith-in the field of view.

(2) Provision of an improved spark discharge device having at least twoseparate sets of electrodes for produc ing arcs under controlledatmospheric conditions.

(3) Provision of Kerr cell control circuits which may be used inconjunction with spark illumination or illumination by conventionallight sources.

(4) Provision of means for producing a point source of light which isautomatically synchronized with the operation of a Kerr cell.

The novel features that are considered characteristic of the inventionare set forth in the appended claims; the invention itself, however,both as to its organization and method of operation, together withadditional objects and advantages thereof, will best be understood fromthe following description of specific embodiments when read inconjunction with the accompanying drawings, in which:

FIGURE 1 is a perspective view of a complete installation of a Kerrcell, pulse generator and associated components arranged for taking ashadowgr-aph picture of a high velocity projectile in a ballistic testrange;

FIGURE 2 is an exploded view of a Kerr cell showing its internalconstruction;

FIGURE 3 is a diagram of a trigger pulse circuit in association with apulse generator and spark discharge device;

FIGURE 4 is a longitudinal cross-sectional view taken on plane 4-4 ofFIGURE 5 showing a spark device which may be used in conjunction withthe circuit of FIGURE 3;

FIGURE 5 is an end elevational view of the spark discharge device shownin FIGURE 4, the end view showing the concentric electrode for producingillumination of the subject at the time that it is photographed;

FIGURE 6 is a Lissajou diagram of elliptically polarized light wavesfrom a Kerr cell;

FIGURE 7 is a Lissajou diagram of plane polarized light waves from aKerr cell; and

FIGURE 8 is a schematic representation of a pulse generator in circuitwith a Kerr cell.

KERR CELL PRINCIPLES Sinch 1875, when Kerr first demonstrated theelectrooptical effect, Kerr cells have been used as optical light valvesor shutters. This type of shutter operates on the principle that certainnormally isotropic substances such as water, nitrobenzene, and carbondisulphide, become optically anisotropic when subjected to electrostaticstress by application of a potential difference to suitably arrangedelectrodes. The electro-optical properties of the Kerr cell are due towhat is known as the Kerr effect, which is the effect of an electricalpotential upon a substance having individual molecules which possessaeolotropic optical polarizability with reference to a set of axeswithin the molecule or a permanent dipole moment, or both. Uponapplication of a strong electric field to such a substance, themolecules assume a difinite orientabine to form an 'el'li'ptica'llypolarized emergent wave.

tion due to the permanent dipole moment. The substantially regulararrangement of molecules cause the substance to exhibit an over-allmolecular asymmetry and optical anisotropy. Although electricalpolarization alone I will cause optical anisotropy, molecularorientation accounts for most of the Kerr effect in substances having ahigh Kerr constant.

With regard to optics, the significance of the Kerr effect is that amedium exhibiting such properties will, when subjected to a strongelectrical field, become doubly refracting or birefringent. Materialspossessed of this property transmit radiant energy, such as lightenergy, at different speeds, depending upon the plane of vibration ofthe energy relative to the field. In other words, the materials areanisotropic when 'electrostatically stressed, having differentproperties in different directions. Thus, an anisotropic medium hasindices of refraction which depend upon the plane of polarization anddirection of propagation of the light waves passing through the medium.

The typical Kerr cell incorporates a pair of spaced plates, which may beelectrically charged, in a medium which exhibits the Kerr eflect.Assuming that N is the index of refraction of light waves travellingperpendicular to the applied field with the plane of polarizationparallel to the applied field and N is the index of refraction for lightWaves travelling perpendicular to the field with the plane ofpolarization perpendicular to the field, Kerr established the followingrelationship:

where N and N are the above defined indices of re fraction, B equals theKerr constant, E is the applied fieldin esu, and A is the wave length ofthe incident light in a vacuum. i

If it is assumed that plane polarized light enters such an anisotropicmedium with its direction of propagation perpendicular to the field andwith its electric vector oriented at some angle qb with respect to theapplied field, the light wave may be resolved into ordinary andextraordinary components parallel to and perpendicular to the directionof the field. The difference in Velocity of propagation of the parallelcomponent and that of the p rpendicular component will result in arelative phase shift of the components as the wave passes through thelength of the field. The phase shift (d) is a linear function of thelength of the field and may be calculated from the formula I v d= LEwhere L is the length of propagation in centimeters, B

. is the Kerr constant, and E is the applied field in esu.

, emergent Wave is plane polarized.

FIGURE 6 illustrates elliptical polarization. In this figure, Xrepresents the extraordinary component which is phase shifted byapproximately 1r/4 radians relative to the oridinary wave 0. Thisfigure, cons'tialcted by Lissajous mechanics, illustrates thattheseWaves com- If voltages proportional to be :the component waves, areimpressed on the horizontal and vertical deflection :plates of a cathoderay tube, an elliptical pattern such as P results on the face of thetube.

In FIGURE 7, X represents the extraordinary component which is phaseshifted by qr radians relative to the ordinary component, .Qfzresultingin plane polarization of the emergent-wave from the Kerr cell. This isindicated by the linear trace P determined by Lissajous mechanics. Asirnilar trace would-appear on the face of a cathode ray tube whereindeflection of the electron beam is proportional-to the component waves.

v and the analyzer.

From the foregoing it Will be apparent that if a polarizer is orientedso that the light entering a Kerr cell is plane polarized with itselectric vector at an angel of =45 with the applied field across theKerr cell, and an analyzer is positioned after the Kerr cell with itsplane of polarization oriented at with respect to that of the polarizer,there must exist a combination of length and applied voltage for theKerr cell which will produce a phase shift 61:71- radians. Such phaseshift results in the emergent beam being plane polarized in the plane ofthe analyzer as can be readily shown by Lissajous mechanics. For a givenvalue of L, the necessary potential V in volts may be readily determinedfrom the foregoing equation, hearing in mind that the field strength isproportional to the gradient of the voltage'across the plates. Thus:

where D the distance between the plates in centimeters.

During the time the field is-applied, the eifective rotation of theplane polarized wave by the Kerr'cell perm-its light to pass from thepolarizer, throughthe Kerr cell Upon interruption of the field, the Kerrcell becomes optically isotropic and no light will pass through theanalyzer, since it is crossed relative to the .polarizer. Hence, such anarrangement may be used as an optical shutter.

A consideration of the foregoing equations will make it apparentthat thevoltage must increase linearly as the distance D between the platesincreases. Because of the unavailability of high potential sources,Ker-r cells 'in the past had very small apertures or narrow'spacingbetween the plates and were therefore of limited utility. By applying avery large voltage to the plates, it is possible 'to provide spacingequivalent to a large aperture, or f number in photographic terms. Thisobviously increases theuti'lity of the Kerr (yell forextremely highspeed photography. .sWith .wide plate spacing, it is possible to usealong focal length lens capable of producing a large image at the filmplane. 'In this way the poor resolution of fast .films :can be olfsetsomewhat.

Wide plate spacing dictates that the voltage must be quite high or thelength of the cell ,must 'bemade quite 'long. Lengthening the cellincreases absorption of light rays within the cell and adverselyaffectsgthe angle of view of the light system. It is therefore desirableto keep the length as short as possible, consistent'with reasonablevaluesof applied voltage.

Nitrohenzene is widely used in Kerr-cells because of its relatively highKer-r constant (346.-0 i=0- esu). As will be evident from the lastequation, this also minimizes the amount of voltage necessary to producethe necessary phase shift within the cell. Electrical conductancethrough nitrobenzene is sufiicicntly small that it may be neglected andthe Kerr cell appears as a pure capacitive load on the circuit used todrive it.

Assuming that it is desirable to use a camera lens having a focal lengthof 5" with ,an aperture of H35, separation of 3.5 centimeters would berequired "for an assumed plate length equal to 10.5 centimeters. The

capacitance C of the plates can be determined from at frequencies aboveI'm/second), A-is the area of one plate in c'm. and b is thedistancebetween the plates in centimeters. For the'assumed values, thecapacitance of the plates will be 28 ,u if. and the required voltage to.

produce the desired phase shift will be approximately 40 kilovolts.

Shown in FIGURE 8 is an equivalent circuit for the Kerr cell K and apulse generator G for driving the cell. If the output of the generatoris a square-wave voltage pulse of duration 5 l seconds, a suitableinternal resistance R of the generator may be defined as that resistancewhich, when combined with the capacitance of the Kerr cell, will producea charging or discharging time constant equal to one-tenth of the pulseduration. For the Kerr cell whose proportions have been assumed above,the time constant T =RC=.5 l0" Substituting the value of the Kerr cellcapacitance, the internal impedance or resistance of the pulse generatorwill be found to be approximately 128 ohms.

The foregoing numerical values are merely representative and should notbe construed as limitations of the invention. By applying an 80 kilovoltvoltage pulse to a cell having 2-inch long plates spaced at 2 inches, anexposure time of l() second can be obtained. This ap proaches thepractical operating limit of a Kerr cell having nitrobenzene since thetime for such fluid to assume its birefringent characteristics afterbeing subjected to a voltage pulse is approximately 1() second.

GENERAL DESCRIPTION Attention should now be directed to FIGURE 1 whichshows a complete installation for taking a picture of a high velocityprojectile 1 fired from a projectile launcher, generally designated 2.The projectile travels along path 3 through a light screen assembly,generally designated 4. This assembly includes a low power light source5 of approximately 25 watts output which projects a thin screen oflight, indicated at 6, toward a photocell 7. The screen, which intransverse section may meaure .1 inch thick and 4 inches wide, ispositioned transversely of path 3 so that the projectile will interruptthe light screen and modulate the intensity of light received by thephotocell. This results in a pulse which is amplified and supplied byconductor 8 to a trigger pulse circuit, indicated generally at 9. Thiscircuit delivers a trigger pulse through conductor 16 to the sparkdischarge assembly and pulse generator, generally designated 11,resulting in discharge of electrical energy formerly stored in the pulsegenerator. Projectile 1 is illuminated by light rays 12 emanating fromthe spark discharge.

Simultaneously, a voltage pulse, which may be as high as 80 kilovolts,is delivered through conductor 13 to the plates 14 of a Kerr cellassembly, generally indicated at 15.

The illuminated projectile can either be directly photographed orphotographed by shadowgraph techniques. The latter offer the advantageof recording the shock waves and wake associated with the projectile.The installation of FIGURE 1 is arranged for shadowgraph photography.Thus, the light rays 12 are collimated by condensing lens 16 forilluminating projectile 1. The objective lens 17 is focused on the rearface of the screen assembly at which plane a shadow of the projectileand its attendant shock wave is formed. The resulting shadow picture isfocused by the objective lens on the film plane 18 of camera 19.

Light rays, in passing from the objective lens 17 to the film plane 18,pass through the Kerr cell assembly which comprises a polarizer 20attached to the front of the assembly, the Kerr cell proper with plates14, and an analyzer 21, attached to the rear of the assembly.

Prior to interruption of the light screen 6 by the projectile, theplates of the Kerr cell are at zero potential and the plane polarizedlight from the polarizer 20 is almost completely blocked by analyzer 21.Athough the Kerr cell transmits a very small amount of light when it isnot energized, the light rays of the light screen do not reach the filmplane since they are directed at 90 to the axis of the Kerr cell. Careis taken to prevent dust parti- .material. is interposed between theseparts to provide a liquid seal. .Optically flat glass end plates 28 and29 are attached optically flat sheets of glass. tail since they areconventional and well-known in the art.

'6 cles from entering the screen assembly and diffusing the light fromthe light screen.

During most of the time that the high potential pulse is applied to theplates of the Kerr cellsome of the pulse period is required to orientthe molecules within the cellthe plane polarized light from polarizer 20is etfectively rotated so that it will pass through analyzer 21 andexpose the film at the film plane 18. Since the high voltage pulse is ofshort duration, the exposure of the film is accomplished in a short timeperiod which may be in the order of 10 second.

Camera 19 may be a conventional camera with a mechanical shutter whichis opened just prior to firing of the projectile launcher and is closedafter the film is exposed. In itself, the mechanical shutter does nottake an active part in taking the high speed photograph.

DETAILS OF KERR CELL Attention is now invited to FIGURE 2 which showsstructural details of the Kerr cell assembly. The Kerr cell propercomprises a tube 22 of heat-resisting, sodaalumina-borosilicate glasswithin which are positioned the plates 14 to which the voltage pulsefrom the pulse generator is applied. The plates themselves are made frompure copper and are spaced parallel to each other within a tolerance ofabout $001 inch. The surfaces of the are electrically connected to acommon junction point 24.

The leads are sealed to glass extensions 25 projecting from the sides ofcylinder 22. In this way the entire assembly is rendered liquid-tight.

To simplify attachment of the tungsten leads to the copper plates 14, anintervening section of nickel (not shown) may be provided. Themachinability of the nickel permits threaded connection to the plates,the nickel sections being welded to the tungsten leads.

The ends of the cylinder are bolted to supports 26 which may be madefrom phenolic plastic or any other suitable At the time of assembly asilicone gasket 27 to the fore and aft supports 26. These are alsobolted in place with suitable gaskets.

The polarizer 20 and the analyzer 21 are directly attached to theexterior faces of the end plates 28 and 29, respectively. Both thepolarizer and the analyzer are made from positive dichroic sheetmaterial, sold under the trademark Polaroid-type HNZZ, mounted betweenThese are not shown in deanalyzer causes transmission to drop to 6% ofthe incident light since the fluid within the cell absorbs a smallamount of light.

From the standpoint of light transmission it would be desirable, ifpractical, to utilize Nicol prisms instead of the Polaroid material.However, at the present state of the art, these Nicol prisms would beprohibitively expensive for a Kerr cell of the physical dimensionsshown.

The Kerr cell assembly is completed by being filled with highly purifiednitrobenzene through filler cap 30.

SINGLE TRIGGER PULSE CIRCUIT FIGURE 3 shows schematically a circuit forsupplying a trigger pulse to a spark discharge device in association 7secondary 32 of the transformer is connected with plates 33 of a doublediode rectifier tube 34. Winding 35 of the transformer supplies currentto directly heated filament 36 of the diode which serves as a full waverectifier in conventional manner. Another winding 37 supplies current toheater 38 of thy-ratron tube 39.

The thyratron tube is a type 2D2l gas filled tube ineluding plate 40,screen grid 41, control grid 42, and cathode 43. The plate 40 isconnected through resistor 44 to conductor 45 which may be regarded asthe 13+ supply.

Grid 42 isbiased relative to cathode 43 through grid resistor 46 andpotentiometer 47 to hold the tube below cut-off.

The resistor of the potentiometer 47, in cooperation with the condenser48, forms the filter for the grid bias supply, whereas condenser-49serves to-filter the 13+ supply. Resistor 50 is a bleeder to improve thepower supply voltage regulation.

'Thethyratron, which is normally non-conducting, may be triggered bymanually closing normally open switch 51. This will charge condenser '52to B+ potential and will feed a positive voltage transient throughblocking condenser 53 to controlgrid 42. In this way thyra tron 39 willbe rendered conductive, and the energy stored in condenser 54 willdischarge via the primary '55 ofpulse I transformer '56 and conductors57-59. The resulting pulse in the secondary '60 'ofthe pulse transformeris applied to trigger electrode '61 whichis centrally positioned andinsulated from spark gap electrode 62.

If desired, the thyratron maybe triggered by a' positive pulse appliedat 63 to the grid circuit of the thyratron. Triggering is accomplishedin this manner when the circuit is used in conjunction with a photocelland amplifier circuit, such as schematically illustrated in FIGURE '1.Such portions of the circuit, being-conventional and'wellknown, have notbeen illustrated.

After switch "51 is opened, the chargeon condenser 52 equalizes throughresistor 64a. The thyra-tron falls below cut-off by current starvationas condenser 54 discharge suificiently to drop the potentialof plate 40below the ionizing potential.

The overall functionof the circuit is to supply a positive trigger pulseof 15,000 volts to trigger-electrode 61. V

This initiates ionization 'in the gap between spark discharge electrodes62 and 64, triggering release of energy which was stored in the pulsegenerator, generally-desigated 65,'by-connecti0n-of a-large negativepotential E to charging resistor 66. It issu flicient to understand atthe moment that the pulse generator is charged with -l1igh potentialenergy whic'his released by the spark discharge betweenelectrodes 62 and64. This establishes a large potential drop across resistor 67 which isgrounded at 68. A large potential drop is also established betweenelectrode 62 and annular electrode 69, resulting in a second andsubstantially simultaneous discharge of energy i betweenthese latter twoelectrodes which is visible through the center of the annular electrode69 and supplies the illumination tor the subject being photographed.

PULSE GENERATOR A pulse generator which has been found remarkably,efiective comprises a single loop of type zRG8/U cable having twoparallel .legs. Thefree endsof the cable 76 and7'1 are positioned.adjacenteach other with the bight Thecable, which nected to the platesof a Kerr cell, indicated diagrammatically at 84. It will be noted thatthe shielding is stripped away exposingthe insulation at and 8.6.

It can be established both by calculation and :by experiment thatasinglesquare wave pulse can be delivered to the plates of the Kerr cell ifresistor 81 is made equal to twice the characteristic impedance of thecable. Al-

"though :the characteristic impedance of 'RGS/U cable is 52 ohms, whenparalleled the characteristic impedance is 26 ohms. lHence, v:resistor3-1 should theoretically be made-.equalit'o 52 ohms in order to drivetheKerr cell with a single :voltage pulse. Using parameters of suchproportions, all other voltage pulses-reflected from the .ends of thecable and the resistor willmutuallycancel,

as Will now bexexplained.

:Before applicationof the trigger pulse to electrode 61, the pulsegeneratoris charged to a ,high :potential by a conventional .50:kilovo'lt, L2 vmilliampere power supply through charging :resistor66.The: charging-potentialmay lie :in the .range 35-50 :kilovolts:depending upon the proportions :of the .Kerricell. When fully charged,energy is stored in the pulse generator by stresses in thediaelectricabetweenithe conductorJandshie'lding of thecable. When thespark dischargeiis triggered between electrodes 62 and 64, a voltagepulse of magnitude +E (a sign reversal may be regarded as resulting fromthe closing of switch 51) travels from the .free :ends along each leg 0fthe cable to'the center region Jof the transmission line. Here, thevoltage pulse encounters a discontinuity of impedance resulting in areflected wave front of +l /s-E ibeing reflected along .each :leg of thecable toward the free ends. One-third of theoriginalpulses aredissipated in resistor 81. The remaining one-third of the :pulses travelalong eachrleg .0f..the cable :to Ibight 72 where they are reflectedwithout :change of sign .to --form two wave fronts of +%E travellingalong eachaleg-of the .cableback toward the loadresistor 81. Bythetirne-thesewave fronts -:reach the loadresistontheother reflectedwave fronts do also, having reached-the freeends of thecableand been.reftcctedwitha reversal-of sign so that.the.reflected wave fronts ofidentical magnitude .-meet at .the load resistor wherethey mutuallycancel .each other.

Itihas been seen that, whenzthe impedance of the load resistor is equal.to twice the'characteristic impedance of .the parallel .cable.generator, aqsingle'voltage pulse'of +E is applied to the platesoftheKerr cell for a time duration equal to;that ne'cessary for'the wave:fronts to travel from ztheresistance'slto the free ends of the cableandbackzto .the resistor. Thus pulse duration is a function of cablelength. For a pulse duration of .01 microsecond, it

a "is recommended that the length :of the generator ,from

.the :free .ends to the bight be 7 'feet and that the load resistor bepositioned inthe center of thislength.

Advantages obviously can be gained by delivering a square wave pulselargenthanE to the Kerr cell; It has .been found that a pulse equal to1- /3E can be delivered by the pulse generator even though it is onlycharged .to a potential of -'E if the load resistor is increased from 52to 100301111118. This, however, results in wave front reflections in the"generator which are not totally cancelled. :In fact, usinga loadresistor of ohms, an uncancelled wave front of %E will be applied totheKerr cell sometime atterthe main pulse of 71 /3 E.

The provision of bight "72 is important. Provision of the bight insteadof free-ends makes is possible to avoid undesirable high voltage coronaeffects. Further, the transmission ;line pulse generator is renderedless susceptible to variations due to humidity and other atmosphericefi'ects. Theover-all result is a'substantial improvement inthe over-allconsistent operation of the generator.

After ;the trigger pulse is appliedto the electrodes, approximately .01microsecond elapses before ionization is complete and the arc of thesparkdischarge .is fully established. As the arc .is established,voltage pulses travel along the legs of the pulse generator until .they

encounter the impedance discontinuity. It is at this time, approximately.005 microsecond after the arc is established, that the square wavepulse is first applied to the Kerr cell. Duration of this pulse willdepend upon the proportions of the pulse generator, as has beenexplained. However, for high speed photography a duration ofapproximately .01 microsecond is desirable. Since the time necessary toorient the molecules of the fluid within the Kerr cell is relativelysmall, the time during which emergent light from the Kerr cell passesthrough the analyzer is equal for practical purposes to the duration ofthe pulse applied to the Kerr cell.

Illumination of the subject begins as the arc is established between thepair of electrodes and is maintained at peak intensity during the timethat light entering the Kerr cell passes through the analyzer. Since theemanation of light from the arc occurs over a time interval of above .2microsecond, it will be apparent that synchronization of Kerr cell andsource of illumination presents no problem and synchronization is fullyautomatic.

SINGLE TRIGGER SPARK DISCHARGE DEVICE FIGURES 4 and 5 disclose thestructure of the spark discharge device including electrodes 62, 64 and69, in

addition to the trigger electrode 61.

of the center section 90 is threadedly engaged with a metal ground ring93 which is attached, as by bolts 94, to an end assembly, generallydesignated 95. Another seal ring is provided at 96 between the end ofcenter section 90 and the end assembly 95 to make the entiredevicegas-tight.

Electrode 64 is cylindrical in form and includes an integral flange 97which is attached by screws 98 to the end assembly. Concentric with andspaced from this electrode is an electrically conductive pilot 99 whichis closely fitted within cylindrical opening 100 of the end cap 91. Thepilot includes an extension 101 with which electrical connection is madeby banana connector 102.

tion 115 of the end plate.

A spring 116 is compressed by the window 114 against the end of theannular electrode 69 and serves to force it against the insulator 108,

'which in turn is forced against the end '107 of the auxiliaryelectrode.

It has been tound convenient to make electrodes 62 and 6-4'and-pilot 99from brass. The trigger electrode may be made from steel. The auxiliaryelectrode 106 'rmay be made from tungsten while the annular electrodecuit of energy from plug 105 to end plate 112.

Attention is now invited to banana connector 117. To simplify theillustration, only one such connector has been shown, although it shouldbe understood that another similar one is provided out of the plane ofthe drawing. These connectors establish an electrically conducting pathbetween the ends 73 and 74 of the pulse generator (see FIGURE 3) totheelectrode 64.

The pulse generator is charged with electrical energy through conductor102, pilot i100, charging resistor 66, and electrode 64. Thisestablishes the very large potential dilference, in the order of tokilovolts, across the gap defined by electrodes 62 and 64. When thetrigger pulse is delivered to the electrode 61, ionization of gasbetween the electrodes 62 and 64 is initiated, resultin g in a suddendischarge of energy as an are between the electrodes. This produces alarge potential dilference across resistor 67 (see FIGURE 3) and causesa second spark discharge from the auxiliary electrode 106 to the annularelectrode 69. The radiant energy produced by the second spark discharge-is transmitted through center opening 118 and the window 114 and servesto illuminate the object being photographed.

metal end plate 112 which is grounded. Since electrode Electrode 64 andpilot 99 support and make electrical I connection with the metallic filmcharging resistor 66 which is cylindrical in form. The resistor includessilver plating at its end surfaces through which electrical connectionis made with the electrode and pilot.

It should be noted that the electrode 62 is hollow and houses acylindrical insulator 103 within which the trigger electrode 61 isimbedded concentrically with the electrode 62 in such manner that theexposed ends of the electrodes define a smooth spherical surface.

Conductor 104 extends through one side of the end assembly and makesconnection with the trigger elec trode 61. This conductor conveys thetrigger pulse from the secondary 60 of the pulse transformer 56 (seeFIG- URE -3) to trigger electrode 61.

A flanged plug 105 is threaded inside of the electrode 62 to confine theinsulator 193. This plug, like the electrode 62, is made vfromelectrically conductive material and supports an auxiliary electrode 106having a conical pointed end .107. A small cylindrical insulator 108 isfitted within opening 109 of plastic pilot washer 110 which surroundsplug 105. Insulator 108 holds the auxiliary electrode *106concentrically in place.

Attention is called to a small cylindrical passage 111 formed ininsulator 108. This communicates with annular electrode 69 which isconcentric with and surrounds insulator 108. This electrode is also madefrom electrically conductive material.

The annular electrode and immediately associated parts are supported bya steel end plate 112. Threadedly engaged with the end plate is aferrule 113 which clamps an optically flat window 114 againstcylindrical projec- 69 is a close fit within the end plate, it also isgrounded. Conductor is shown rotated out of its true angular position(see FIGURE 5) to simplify the illustration.

In order to prevent spontaneous discharge prior to application of thetrigger pulse, a controlled atmosphere at about 5 p.s.i.g. is providedin the gap between the electrodes 62 and 64. This atmosphere, which maybe freon gas (CCI F is introduced to the gap and to the regionsurrounding resistors 66 by passages 1-19 and 120.

A controlled atmosphere is also provided between the electrodes 106 and69. This atmosphere may consist of xenon, argon, or some other inertgas, introduced through passage 121. By using a charging pressure of 600to 800 p.s.i.g., not only spontaneous spark discharge is prevented butillumination efiiciency is increased through the increased density ofatoms in the spark gap which are subjected to light emitting changes ofenergy level in their electron rings.

Seal ring 122 is provided on pilot 99 and similar seals are provided at.123 about electrode 64. Presence of other seals 124- 127 will also benoted in positions which render the entire device gas-tight.

A ground strap 128 interconnect-s the shielding (not shown) of theconductor 66a (which conveys energy to conductor 102) to the ground ring93. Another ground strap 129 interconnects shielding 7-5 of thetransmission line generator with the ground ring 93. A similar groundstrap (not shown in FIGURE 4) also interconnects shielding 76 with theground ring. (Shielding 75 and 76 are indicated diagrammatically inFIGURE 3.) It will be understood that the ground ring is connected byseparate means (not shown) to the ground connection of the circuit shownin FIGURE 3.

eaeseee :In partial summary, it will he noted that a device is providedhaving asingle trigger electrode or initiating spark discharge betweenelectrodes 6} and 4. "The re- .sulting discharge -ofcnergy causes asecond spark discharge between electrodes 106 and 69 proyidingillumination im the subject being photographed. Both spark dischargesoccur substantially simultaneously 9 trolled atmospheres.

Shown in FIGURE '5 is the .end Niew of theispark discharge device.Particular note should be of the window 114 nd th s in 1 8 mush whi li hilluminating the subject passes. v

' PARAMETERS,

The following parameters, although not. limitations of the invention,:have been used in the :foregoingcircuit of:

FIGURE Transformer 3;]. m fewer transformer 20,600 ohms, 2 watt.

so re, so volt 20 gfd, sou var o. 220,000 ohms, 2 watt.

Potentiometer Condenser 48 H Condenser 49 Various other features andadvantages not .sp fically wise will many variations :and:mQ'd'ifiCatinhs @Qf zthfl P P- vferrecl embodimentofzthe invention,18.11 of whichmay be achieved without departing-tram :the spirit and;.scop.e :of the invent-ion. a

Havingdescribed-rny invention, :1 claim: 1. A spark discharge devicezcomprising a'itilhular se ction supportedibetweendnsulating 1 d zmemlqels, L311 relc tricall-y conductive pilot supported by one iofsaid endmembers, atcombined pilot and electrode supported by said other 'endmember, .a tubular rre'sistor :;suppor;ted by said pilot and saidelectrode a nd ,rnakir g;electrical connection therewith, means forrdcliyening potential energy to said pilot, a means zfQl' storingelectrical e'n- 'ergy connected to ,said eelectrodega second eelect 0 rajacent {to but spaced from said f rst teleptrode, a" gggef electrodeinsulated from and concentric @with said ,sec-

The various features and advantages ,of'rthe invention :40

.are thought to be clear from the foregoing descriptionsQldd electrodefor establishing discharge of electrical ene r gy from said storagemeans: between said electrodes,

an auxiliary electrode connected to said second electrode,

ananuular electrode spaced from said auxiliary elec- 5 tr.ode, aresistor interconnecting said second electrode and ground, and means forgrounding said annular electrode, said annular electrode defining anopening through which spark discharge between said auxiliary electrodeand said annular electrode is visible.

'2. Apparatus as defined in claim 1 and, in addition, means forproviding a controlled atmosphere in the gap between said first andsecond electrodes and a different controlled atmosphere between saidauxiliary and said annular electrodes.

3. A spark discharge device comprising a tubular section supportedbetweeni ns ilating .end members, an electrically conductive pilot.,supported by one of said end memhers, a combined pilot and electrodesupported by said other end member, a tubular resistor supported by saidpilot and said electrode and mflking'electrical connection therewith,means for delivering high potential energy to said pilot, a secondelectrode adjacent to but spaced ,from said ,first electrode, capacitiveload means for storing electrical energy connected between said firstand second electrodes a trigger electrode insulated O $1-9Q entric withsaid second electrode it'or establishing discharge of electricalenergyfrom said vstorians bet ee -sai e ct ode the expbsed en s of sard \s endand trigger electrodes defining ,a smooth .als rface, anauxiliaryelectrode having a conical d tend vconnected to said second electrode,an andrical insulatorisuppor in g the conical end of said iary elc rodeconcentrically with the aperture of iahnular e ec v-q and ha in a cyl ndc l p a defining the gap between said annular andaujdliary electrodes aresistor interconnecting said second electrode and groun fand means torgrounding said annularielec- .tr odegsaid annularoelectrode defining anopening through which spark discharge between said auxiliary electrodeand said annularclectrode is visible.

Refe ences Cited .brthe Examiner UN TED STATES P TENT 5/1957 Miller 3 1s241 /1957 Holliday sis-241 4/l-959 wagner '3=l5 241 5/1960 Nolan a1-s-241 .OIHER REFERENCES ilil ctronica lv tCv -nltzrvo ll-edtspectrographic Spark -.Source;:Nature,1une 27, 1953, page 1156.

JJEQHN y -flr m z yflxemiu r- JAMES D. KALLAM, 'DAVJD -J. GALYIN,

- Examiners electrode spaced from said auxiliary electrode, a

1. A SPARK DISCHARGE DEVICE COMPRISING A TUBULAR SECTION SUPPORTEDBETWEEN INSULATING END MEMBERS, AN ELECTRICALLY CONDUCTIVE PILOTSUPPORTED BY ONE OF SAID END MEMBERS, A COMBINED PILOT AND ELECTRODESUPPORTED BY ING, A SPRING FOR BIASING THE HOUSING OUTWARDLY IN THE SAIDPILOT AND SAID ELECTRODE AND MAKING ELECTRICAL CONNECTION THEREWITH,MEANS FOR DELIVERING HIGH POTENTIAL ENERGY TO SAID PILOT, A MEANS FORSTORING ELECTRICAL ENERGY CONNECTED TO SAID ELECTRODE, A SECONDELECTRODE ADJACENT TO BUT SPACED FROM SAID FIRST ELECTRODE, A TRIGGERELECTRODE INSULATED FROM AND CONCENTRIC WITH SAID SECOND ELECTRODE FORESTABLISHING DISCHARGE OF ELECTRICAL ENERGY FROM SAID STORAGE MEANSBETWEEN SAID ELECTRODE, AN AUXILIARY ELECTRODE CONNECTED TO SAID SECONDELECTRODE, AN ANNULAR ELECTRODE SPACED FROM SAID AUXILIARY ELECTRODE, ARESISTOR INTERCONNECTING SAID SECOND ELECTRODE AND GROUND, AND MEANS FORGROUNDING SAID ANNULAR ELECTRODE, SAID ANNULAR ELECTRODE DEFINING ANOPENING THROUGH WHICH SPARK DISCHARGE BETWEEN SAID AUXILIARY ELECTRODEAND SAID ANNULAR ELECTRODE IS VISIBLE.